1. Field of the Invention
The present invention concerns a method for manufacturing a winding tape of unsintered polytetrafluoroethylene wherein polytetrafluoroethylene powder is mixed with a lubricating or slip agent, the material is molded into a cylindrical blank and the blank is first extruded as a strand with small outer dimensions and subsequently calendered into the tape form.
2. Description of the Background Art
A method of this generic type is known, for example, from Swiss patent 562 098. Here, the extruded strand with small outer dimensions is a flat tape with a rectangular cross-section that is calendered to a thin tape practically without changing its width. The length increases by approximately 5 to 20 times its original length in the process. Apart from the fact that protruding edges on the surface cannot be avoided with such a tape when it is used as winding tape, the extrusion of the powder/lubricant mixture in tape form and subsequent elongation of the tape through calendering results in an orientation of the material particles exclusively in the axial or tape direction. This unidirectional orientation of the material can give rise to longitudinal tears in the end product, for example under temperature cycling stress.